This invention relates generally to improved communication systems and methods involving high performance serial data transmission systems, and, in particular, to methods of modifying the characteristics of the message in order to communicate non-standard messages. A standard message is defined in this application as belonging to the most significant category of message traffic carried by the communication system, at least in terms of content. Usually standard messages form the category of message carried most frequently by the system. Normally, such standard messages possess a fixed and predetermined message format. Most often, standard messages have a format defined in terms of length, ultimately measured in the number of bits, but other physical characteristics, such as bit rate, special data patterns as pre- or post-fixes, or dedicated transmission channels, may be used to distinguish standard from non-standard messages.
Examples of common terms for physical message entities are "bytes", "words", "blocks", and "frames". Whether such a term refers to standard or non-standard messages depends in general on some distinguishing physical characteristic, and the interpretation of the message may be entirely different in the two classes. Non-standard messages might be used, for example, for monitoring and controlling the operation of the transmission system or for communicating unusual types of data messages.
In recent years there has arisen a demand for high performance digital data transmission techniques, especially in the fields of information processing, scientific instrumentation, and remote operation of instruments or machines. By "high performance" is meant communication at sufficiently high data rates and over distances sufficiently great that the physical properties of the transmission lines become determining factors in the maximum data transfer speed permissible with insignificant error rate.
Primarily for economic reasons, it is usually important to keep high performance transmission systems as "busy" as possible. In other words the system bandwidth should be fully occupied. Since such systems are typically employed for long distance communications, it is costly to provide additional message channels for monitoring system performance, for system control, or for other non-standard communications. Thus it is common to use the same physical channel media (i.e., transmission lines) for the latter purposes. It is frequently of considerable value to minimize the loss of overall system bandwidth for such non-standard (but often essential) purposes.
The invention disclosed in this application is a novel method for providing for transmission of non-standard messages over the normal message channels. The method is simple, inexpensive, efficient in terms of system bandwidth, and particularly well adapted to data transmission systems using an interrupted clock protocol. Also disclosed in this application is an improved apparatus for regenerating the interrupted-clock timing messages when the novel method of non-standard messages is used.